Downstream Migration of Silver-Phase Anguillid Eels

Abstract

Downstream migration of adult anguillid eels in freshwater habitats marks the end of a long, generally slow growth phase, abandonment of a home range, and initiation of a seaward spawning migration. Although different species of Anguilla exhibit variation in characteristics of this migratory phase, some generalized traits in migration are common to most species. For temperate and subtropical species of Anguilla,migratory periods are usually associated with times of year when water temperatures are decreasing and flows are increasing. Species of Anguilla in the northern hemisphere usually emigrate between the months of August and December, while those of the southern hemisphere leave freshwater habitats between January and March. In the northern hemisphere, A. rostrata tends to emigrate earlier at higher latitudes (Fig. 1). However, Tesch (1977) concluded that the migration date of A. anguilla could not be predicted on the basis of geography alone. Onset of the silver migratory phase is generally morphologically characterized in most species of Anguilla as a darkening of dorsal surfaces and fin margins, silvering of lateral integument, enlargement of eye diameter, and an increase in gonad development (Lecomte-Finiger 1990). Other endocrinological changes also occur during metamorphosis (Durif et al. 2000).